The present invention relates to an imaging system capable of correcting colors of a captured image of an object to correctly display the colors of the image.
Colors of objects are susceptible to illumination conditions, and it is therefore difficult to always display correct colors of images captured by a camera. Human eyes can correctly recognize actual colors of objects regardless of such conditions, which ability is known as color constancy.
Existing video cameras do not comprise imaging devices having this feature. Attempts are being made to implement the color constancy in imaging systems having such video cameras by performing complicated correction, e.g., by comparing color of a particular point with the surrounding color to make correction. However, these attempts are not practical, since they are limited to correction to special images or the image processing takes a long time.
An object of the present invention is to provide an imaging system with good response which can correct colors of taken images of objects at practical speed to correctly display the colors of the images.
The present invention is directed to an imaging system comprising an imaging device for taking a color image and a lens for forming an image of an object on said imaging device, The imaging system comprising a reflection surface, provided within a maximum field of view formed by said lens and said imaging device, for diffuse-reflecting the image of said object to cause the image to impinge upon said imaging device through said lens, assigning means for assigning each main coordinates (Xmi, Ymi) in a direct image obtained as an object point on said object is imaged on said imaging device to corresponding sub-coordinates (Xni, Yni) in an indirect image of said object point obtained from said reflection surface on said imaging device, and a color-correcting portion for obtaining a color-corrected image on the basis of the following expressions.
D1 (Xmi, Ymi)=(Rmi/Rni)xc2x7S,
D2 (Xmi, Ymi)=(Gmi/Gni)xc2x7S,
and
D3 (Xmi, Ymi)=(Bmi/Bni)xc2x7S.
Where D1, D2, and D3 respectively represent R, G, B components of the corrected color image at said main coordinates (Xmi, Ymi), Rmi, Gmi, and Bmi respectively represent R, G, B components in a direct image pixel (Pm) at said main coordinates (Xmi, Ymi), Rni, Gni, and Bni respectively represent R, G, B components in an indirect image pixel (Pn) at said sub-coordinates (Xni, Yni), and S represents a correction term.
The analysis by the inventor described later, revealed that a diffuse-reflected indirect image at the reflection surface provided within the maximum field of view, represents the brightness at the object point. Accordingly, dividing Rmi, Gmi, and Bmi respectively by Rni, Gni, and Bni representing the brightness eliminates errors due to effects of illumination. This was confirmed by experiments carried out by the inventor. The correction term S prevents the output resulting from the division of Rmi, Gmi, Bmi by Rni, Gni, Bni from exceeding the limit of device scale width and becoming saturated.
Particularly, setting said reflection surface so that a direct image section for forming said direct image in said imaging device has a larger width than an indirect image section for forming said indirect image enables effective use of the maximum field of view of the imaging device. Moreover, as will be described later, it was confirmed that the color correction encounters no problem even when the width of the indirect image section is about 25% of the maximum field of view.
It is preferred that the imaging system comprises a cover provided on the side on which light comes into said lens, for intercepting light from outside of said maximum field of view at least. While light outside the maximum field of view causes errors in color correction, the cover reduces the errors.
When designing the reflection surface, the direct image and the indirect image of said object may be similar figures with respect to the direction in which said direct image section and said indirect image section are arranged. In this case, it is possible to corrected color small objects like flowers more precisely to the details.
The reflection surface may be so designed that the ratio of the numbers of corresponding pixels between the indirect image section for forming said indirect image and the direct image section for forming said direct image in the direction in which said direct image section and said indirect image section are arranged is constant. In this case, the algorithm for color correction can be simplified and the color correction can be achieved at very high speed.
The reflection surface can be shaped according to the following equation.
Xni=f(Axe2x88x92tan(2xcex1))/(1+Axc2x7tan(2xcex1))
Where f represents focal length of the lens, A represents (X/Z), X represents horizontal-direction distance of the object point P from a horizontal-reference line Ch, Z represents vertical distance of the object point P from a vertical-reference line Cv, and xcex1 represents an angle formed between the reflection surface and a horizontal line parallel to the vertical-reference line Cv.
Experiments made by the inventor showed that forming said reflection surface with leather having oil coating on its surface allows the color correction to be achieved very well.
The present invention can be implemented by installing software for realizing the assigning means stored in a storage medium into a common personal computer and attaching the cover having the reflection surface to a common video camera.
As stated above, the features of the present invention provides an imaging system with good response which can correct colors of a taken image of an object at practical speed by comparing an indirect image from the reflection surface and a direct image so as to correctly display the colors of the object.
The present invention will become more apparent from the following detailed description of the embodiments and examples of the present invention. The reference characters in claims are attached just for convenience to clearly show correspondence with the drawings, which are not intended to limit the present invention to the configurations shown in the accompanying drawings.